Ping-pong ball avalanches may help prevent real disasters

A series of experiments, some involving over half a million ping-pong balls and a ski slope, are helping scientists understand the complex physics underlying the most devastating type of mountain disaster - powder snow avalanches.

Jim McElwaine, now at Cambridge University, UK, and Kouichi Nishimura, at Hokkaido University, Japan, have conducted a number of tests with real snow but also decided to used ping-pong balls to solve a particular problem.

Avalanches involving more compact types of snow can be simulated using plastic beads. But powder snow becomes airborne during an avalanche, meaning the particles used in simulations must be light enough to behave in the same way. Using table tennis balls was the researchers' answer. In one dramatic experiment carried out near the start of the research programme, 550,000 balls were sent tumbling down a Japanese ski jump.

Powder snow avalanches can be particularly devastating because they behave in a much less predictable way than "dense-flow" types. Once the lighter snow becomes suspended in the air at the front of the avalanche, it can travel much further than might be expected.

"There aren't any good theories that have been verified" for powder snow avalanches, McElwaine told New Scientist. However, the ping-pong ball tests enabled the researchers to develop mathematical models that could describe the behaviour of powder snow avalanches.

McElwaine is now conducting further experiments using real snow at the Swiss Federal Institute for Snow and Avalanche Research in order to test the usefulness of the mathematical models. Such models could help scientists predict the behaviour of avalanches, enabling engineers to plan better defences for mountain villages.

During the ski-jump experiments, which began in 1999, cameras and air pressure sensors were placed at regular intervals along the decline. These instruments allowed the researchers to track the movement of individual ping-pong balls and measure their velocity. The air pressure sensors also revealed turbulent eddies within the bouncing mass of balls.

The mathematical model derived from the results was in fact fairly straightforward. "We were very surprised that such a complex phenomenon could be explained with a relatively simple theory," McElwaine says. But he adds that it could take a few of years of further work to figure out if the model will does accurately describe real avalanches.

Understanding powder snow avalanches is certainly a priority, says Mark Williams, an avalanche researcher at the University of Colorado, Boulder, in the US. "These are the real killers," he told New Scientist. "We'd like to be able to predict these avalanches, and I think this would help."

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Over half a million ping-pong balls were released in the largest test (Image: Hokkaido University)